1. The Field of the Invention
The present invention relates to drug delivery or drug eluting stents. More specifically, the invention relates to methods for producing drug delivery stent coatings which have variable stent coating thicknesses by changing the stent diameter during the application of the coating process, deposition, or formation.
2. Related Background
Stents are frequently used in the health care industry to open vessels affected by occasions such as stenosis, thrombosis, restenosis, vulnerable plaque, and formation of intimal flaps or torn arterial linings caused by percutaneous translumenal coronary angioplasty (PCTA). Stents are used not only as a mechanical intervention, but also as vehicles for providing biological therapy. As a mechanical intervention, stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the passageway wall. Stents may be capable of being compressed, so that they can be inserted through small cavities via catheters or balloon-catheters and then expanded to a larger diameter once they have been isolated at the target location. There are many examples in the patent literature disclosing stent structures which have been successfully applied in procedures including stents illustrated in U.S. Pat. Nos. 4,733,665 issued to Palmaz, 4,800,822 issued to Gianturco, and 4,886,062 issued to Wikor.
One example of a stent, which is designed to expand and contract under radial pressure is illustrated by FIG. 1. Stent 10 is shown to have struts 12 separated by spaces which allow the stent to be crimped on a catheter or balloon and to allow the stent to follow a tortuous pathway of anatomical structures. In lieu of struts, stents can be made from filaments, fibers, rolled up metal sheets, and a multitude of other mechanical designs with unique geometries which can be found in the medical and patent literature.
In addition to mechanical intervention, stents can be modified by medication to provide for local administration of drugs, bioactive agents, or therapeutic substances, terms which are used interchangeably unless otherwise specifically indicated. Medicated stents provide for the local administration of a therapeutic substance at the diseased site or periphery of the site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
The current standard of coating stents includes the use of a polymer matrix coated onto the surface of a stent. Polymer coated stents have been well enabled and described by the patent literature as of the filing date this invention and for the sake of brevity, no prolonged discussion should be merited for those skilled in the art. Briefly, to a stent is applied a polymer dissolved in a solvent with a drug added thereto. Once the solvent evaporates, a polymer film is left behind with the drug embedded or contained therein. In vivo, the drug releases or elutes out from the polymer.
Although drug delivery stents have shown unmatched advances, as with any invention, room for improvement and modifying weaknesses always exists. Perfecting drug delivery stents can be in the type of polymer or drug used, dosages, advances in coating constructs that tailor specific needs of patient care, fine-tuning of the coating topography that can address specific biological deficiencies. For example, thrombosis can be one adverse clinical sequela of stenting. Some drug eluting stents can have higher incidents of late thrombosis compared to bare metal stents. Even though thrombosis is rare, the consequences of late thrombosis are driving some cardiologist to prescribe prolonged, or even permanent, antiplatelet drug therapy. One concern is whether drug eluting stents re-endothelialize more slowly than bare metal stents. Some cell culture and animal study data suggest that some drugs may counter or inhibit endothelial cell proliferation. This effect appears pronounced nearest the surfaces of the struts, where the drug amount concentration is highest and most prolonged. This effect is of most concern on the lumenal surfaces of the stent struts, where the goal is to have rapid and complete reendothelialization after stent placement.
In some circumstances, there may be a need to have a greater amount or concentration of the drug on the ablumenal surface for better management of restenosis, such that only a small amount of the drug is washed down-stream from the luminal coating by the flow of blood. As yet another coating construct alternative, for certain patient sub-set, it may be beneficial to have a higher anti-restenotic concentration of the drug on the ablumenal surface than the lumenal side and higher concentration of a secondary drug, such as anti-clotting agent (e.g., heparin) or growth factor (e.g., angiogenesis drug, VEGF, etc.), on the lumenal side than the ablumenal side for synergistic effects.
Such ability to modify stent coating can lead to better and effective patient management. The methods of the present invention provide apparatus, means and techniques for achieving these as well as other goals that are apparent to one skilled in the art.